Cellular radio systems are used to provide telecommunications to mobile users. In order to meet the capacity demand, within the available frequency band allocation, cellular radio systems divide a geographic area to be covered into cells. At the centre of each cell is a base station through which the mobile or fixed outstations communicate with each other and with a fixed (wired) network. The available communication channels are divided between the cells such that the same group of channels are reused by certain cells. The distance between the reused cells is planned such that co-channel interference is maintained at a tolerable level.
When a new cellular radio system is initially deployed operators are often interested in maximising the uplink (mobile station to base station) and downlink (base station to mobile station) range. Any increase in range means that less cells are required to cover a given geographic area, hence reducing the number of base stations and associated infrastructure costs. The downlink range is primarily increased by increasing the radiated power from the base station. National regulations, which vary from country to country, set a maximum limit on the amount of effective isotropic radiated power (EIRP) which may be emitted from a particular type of antenna being used for a particular application. In Great Britain. for example. the EIRP limit for digital cellular systems is currently set at +56 dBm. Hence the operator is constrained and, in order to gain the maximum range allowable, must operate as close as possible to the EIRP limit. without exceeding it.
One form of layered antenna (an antenna having ground planes, feed networks and dielectric spacers arranged in layers) is known from British Patent GB-B-2261554 (Northern Telecom) and comprises a radiating element including a pair of closely spaced correspondingly apertured ground planes with an interposed printed film circuit, electrically isolated from the ground planes. the film circuit providing excitation elements or probes within the areas of the apertures, to form dipoles, and a feed network for the dipoles. A sectional view of such an antenna is shown in FIG. 1: a frontal view of the first three radiating elements is shown in FIG. 2.
The array antenna is constructed of a first apertured metal or ground plane 10, a second like metal or ground plane 12 and an interposed film circuit 14. Conveniently the planes 10 and 12 are fiat, thin metal sheets, e.g. of aluminium, and have substantially identical arrays of apertures 11 formed therein by, e.g. press punching. In the embodiment shown the apertures are rectangular and formed as a single linear array. The film circuit 14 comprises a printed copper circuit pattern 14a on a thin dielectric film 14b. When sandwiched between the apertured ground planes part of the copper pattern 14a provides probes 16, 18 which extend into the areas of the apertures. The probes are electrically connected to a common feed point by the remainder of the printed circuit pattern which forms a feed conductor network in a conventional manner. In the embodiment shown the totality of probes in the array form a vertically polarised antenna when the linear array is positioned vertically. In a conventional triplate structure the film circuit is located between and spaced from the ground planes by sheets of foamed dielectric material 22. Alternative mechanical means for maintaining the separation of the feed conductor network may be employed, especially if the feed network is supported on a rigid dielectric. Referring now to FIG. 2, the linear array comprises of a number of radiating elements 201 which have radiating probes 216 and 218 oppositely directed within aperature 210.
In order to increase output from the antenna in a primary radiating direction, the antenna may further comprise a further ground plane placed parallel with and spaced from one of the apertured ground planes to form a rear reflector for the antenna. Signals transmitted by the antenna towards the back plane are re-radiated in a forward direction.
Typically, for a cellular wireless communications base station, there is a linear arrangement of a plurality of spaced apart antenna radiating apertures/elements to form a linear array. It is often the case that an m x n planar antenna array is constructed from m linear arrays having n radiating apertures spaced at regular intervals. In cellular radio base stations, the antennas are generally arranged to cover sectors, of typically 120.degree. in azimuth--for a tri-sectored base station. Each vertically oriented antenna array is positioned parallel with the other linear antenna arrays. The radiating antenna elements of a vertical array co-operate to provide a central narrow beam coverage in the elevation plane and broad coverage in azimuth, radiating normally in relation to the vertical plane of the antenna array. In the elevation plane the radiation pattern consists of a narrow "main" beam with the full gain of the antenna array, plus "side lobes" with lower gains. This type of antenna lends itself to a cheap yet effective construction for a planar array antenna.
Downtilt in the cellular radio environment is used to decrease cell size from a beam shape directed to the horizon to the periphery of the cell. This provides a reduction in beam coverage, yet allows a greater number of users to operate within a cell since there is a reduction in the number of interfering signals.
This tilt can be obtained by mechanically tilting the antenna array or by differences in the electrical feed network for all the antenna elements in the antenna array. Mechanical downtilting is simple but requires optimisation on site and can only provide a physical tilt, i.e. the beam shape with respect to the antenna is not changed; electrical downtilting allows simple installation and is a slightly more complex design. Electrical downtilt can be used to direct a radiation beam downwardly from an axis corresponding to a normal subtended by an array plane to form a conical beam pattern which provides an ideal coverage, especially in the case of tri-cellular antennas. The downtilt results from a consecutive phase change in the signal fed to each antenna element in an antenna array, i.e. the antenna can be said to have a progressive phase feed network. Typically, a downtilt of 2.5.degree. or 5.degree. is employed. but this can vary depending on the terrain local to a base station.
This progressive phase change (n.degree.), however, introduces cross polar radiation currents (CPRC), as can be seen in FIG. 3, which can be compared with a non-steered flat plate antenna (i.e. having no progressive phase difference between the radiating elements). Cross polar radiation currents in turn provide gain associated with such cross polar radiation currents, and this reduces the required gain of the antenna in the azimuth direction. FIG. 4 provides a graphical representation of a loss in gain across a portion of the band attributable to cross-polar radiation.
Careful design of the dimensions of the apertures and the elements coupled with the design of the electrical characteristics of the feed network for the elements can control the cross-polar radiation to some extent, but this is not wholly effective. The Applicants have determined an antenna array providing electrical downtilt (whereby the feed network provides a progressive phase distribution for the radiating apertures), cross-polar radiation levels at resonant frequencies arise in the apertured ground planes which reduce the gain in the operating frequency band.